Advanced multi-layer active magnetic regenerator systems and processes for magnetocaloric liquefaction

ABSTRACT

A process for liquefying a process gas that includes introducing a heat transfer fluid into an active magnetic regenerative refrigerator apparatus that comprises a single stage comprising dual multilayer regenerators located axially opposite to each other.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/477,924, filed Mar. 28, 2017, which is herein incorporated byreference in its entirety.

ACKNOWLEDGMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Contract No.DE-AC05-76RL01830 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

BACKGROUND

Broad use of hydrogen as a fuel or energy carrier will provide betterenergy security, return major economic, environmental, and healthbenefits, and help minimize climate-change impact of greenhouse-gasemissions from energy use. Hydrogen couples into any realistic model of“sustainable carbon-hydrogen-electricity cycles” in an integrated andcritical manner.

For storage and delivery, liquid hydrogen (LH₂) is the superior choicerather than compressed (CH₂), adsorbed, or chemical compounds ofhydrogen because of LH₂'s higher volumetric energy density andgravimetric energy density compared to other hydrogen storage methods.The ratio of the ideal minimum work input per unit mass of gas to thereal work input per unit mass of gas for a practical liquefier is calledfigure of merit (FOM). Currently most gaseous hydrogen (GH₂) isliquefied using liquid nitrogen pre-cooled Claude-cycle plants. Theseconventional large-scale liquefiers are limited to a FOM of ˜0.35.Small-scale conventional liquefiers seldom achieve FOMs of 0.25. Such alow FOM increases operating costs of hydrogen liquefiers and thereby theprice of dispensed LH₂ or CH₂ fuel.

A relatively small number of hydrogen liquefiers presently exist in theworld. Most of them are large industrial plants with capacities rangingfrom ˜5 metric tons/day to ˜100 metric tons/day. Most commercial H₂ hasbeen used for non-transportation applications such as at refineries andammonia fertilizer plants. Few commercial liquefaction facilities havebeen built with capacities below ˜5 metric tons/day because the turn-keyinstalled costs tend to increase sharply on a per metric ton/day basisas the capacity decreases. The depreciation of high capital costs ofhydrogen liquefiers increases the price of dispensed LH₂ or CH₂ fuel.For example, a 1 metric ton/day LH₂ facility has an approximateinstalled cost of ˜$9-11 million, i.e., ˜$10 million/metric ton/day.Over a 20-year operating period of a plant of this capacity and cost,straight-line depreciation gives a contribution of ˜$1.45/kg to H₂ fuelcost.

The major barriers to deployment of fuel-cell electric vehicles are lackof local supply and refueling infrastructure with capacity in the rangeof 0.1 to 1 metric ton/day at each refueling station with delivery ofLH₂ or CH₂ at the same price or less than gasoline on a fuel cost/miledriven basis. Cost-effective and efficient hydrogen liquefiers on thisscale for such refueling supply and refueling stations do not exist.These two key barriers to more rapid adoption of hydrogen fuels can beeliminated by development of highly-efficient and low-cost small-scaleliquefiers.

Active magnetic regenerative refrigerator (AMRR) systems with highperformance magnetic regenerators with multiple magnetic refrigerantsmay be able to efficiently liquefy hydrogen, but there have been severalproblems in developing multi-layer regenerators which are theoreticallycapable of efficiently spanning large temperature ranges required forcryogen liquefier systems such μ280 K to ˜20 K for LH₂. However, inpractice, operational multi-layer cryogenic regenerators spanning overmore than ˜80 K do not exist.

SUMMARY

Disclosed herein in one embodiment is a process for liquefying a processgas comprising:

introducing a heat transfer fluid into an active magnetic regenerativerefrigerator apparatus that comprises dual regenerators located axiallyopposite to each other, wherein the apparatus comprise (i) a first topregenerator comprising 2 to 16 successive layers, wherein each layercomprises an independently compositionally distinct magnetic refrigerantmaterial having an independent Curie temperature and wherein the firstlayer of the top regenerator has the highest Curie temperature and thelast layer of the top generator has the lowest Curie temperature and(ii) a second bottom regenerator comprising 2 to 16 successive layers,wherein each layer comprises an independently compositionally distinctmagnetic refrigerant material having an independent Curie temperatureand wherein the first layer of the bottom regenerator has the lowestCurie temperature and the last layer of the bottom regenerator has thehighest Curie temperature;

flowing the heat transfer fluid through each layer of the first topregenerator and each layer of the second bottom regenerator;

diverting a portion of the flowing heat transfer fluid from an outlet ofeach layer of the first top regenerator to an inlet of the correspondingCurie temperature layer of the second bottom regenerator, except forlowest Curie temperature layer;

diverting a bypass portion of the flowing heat transfer fluid from thelowest Curie temperature layer of the first top regenerator into abypass flow heat exchanger at a first cold inlet temperature;

introducing the process gas into the bypass flow heat exchanger at afirst hot inlet temperature and at a counterflow with the bypass portionflow, and discharging the process gas or liquid from the bypass flowheat exchanger at a first cold exit temperature; and

simultaneously subjecting all of the layers of the second bottomregenerator to a higher magnetic field while all of the layers of firsttop regenerator are demagnetized or subjected to a lower magnetic field.

Also disclosed herein is a system comprising:

a first active magnetic regenerative regenerator comprising 2 to 16successive layers, wherein each layer comprises an independentlycompositionally distinct magnetic refrigerant material having anindependent Curie temperature and wherein the first layer of the firstactive magnetic regenerative regenerator has the highest Curietemperature and the last layer of the first active magnetic regenerativeregenerator has the lowest Curie temperature;

a second active magnetic regenerative regenerator comprising 2 to 16successive layers, wherein each layer comprises an independentlycompositionally distinct magnetic refrigerant material having anindependent Curie temperature and wherein the first layer of the secondactive magnetic regenerative regenerator has the lowest Curietemperature and the last layer of the second active magneticregenerative regenerator has the highest Curie temperature;

at least one conduit fluidly coupled between the lowest Curietemperature layer of the first active magnetic regenerative regeneratorand the highest Curie temperature layer of the second active magneticregenerative regenerator;

a single bypass flow heat exchanger (a) fluidly coupled to the lowestCurie temperature layer of the first active magnetic regenerativeregenerator and (b) fluidly coupled to a process gas source; and

for each layer of the first active magnetic regenerative regenerator andeach layer of the second active magnetic regenerative regenerator, anindependent fluid conduit between an outlet of each layer of the firstactive magnetic regenerative regenerator to an inlet of thecorresponding Curie temperature layer of the second active magneticregenerative regenerator, except for lowest Curie temperature layer offirst module.

Further disclosed herein is an apparatus comprising:

an active magnetic regenerative regenerator comprising multiplesuccessive layers, wherein each layer comprises an independentlycompositionally distinct magnetic refrigerant material having Curietemperatures 18-22 K apart between successively adjacent layers, and thelayers are arranged in successive Curie temperature order and magneticrefrigerant material mass order with a first layer having the highestCurie temperature layer and highest magnetic refrigerant material massand the last layer having the lowest Curie temperature layer and lowestmagnetic refrigerant material mass.

The foregoing will become more apparent from the following detaileddescription, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an eight layer dual active magnetic regenerator(AMR) for active magnetic regenerative refrigeration (AMRR) withcontinuous bypass flow operating between 280 K and 120 K.

FIG. 2 is a cross sectional perspective view of an AMRR apparatus.

FIG. 3 is a cross sectional perspective view of an AMRR regenerator.

FIG. 4 is a perspective view of an AMRR apparatus.

FIG. 5 is a schematic diagram of a cross-section of a rotary wheelembodiment of a single-stage active magnetic regenerative refrigerator(AMRR) with bypass flow. For example, the embodiment shown in FIG. 5 isa schematic diagram of a single stage AMRR with layered magneticmaterials and bypass flow of heat transfer fluid to continuously cool aprocess stream such as gaseous H₂ (GH2) or natural gas. In this stagethere are eight magnetic materials to span from ˜280 K to ˜122 K. Thisdesign also works for 122 K to 53 K or from 53 K to 22 K; each withfewer layers of refrigerants and less material as required for a LH₂liquefier.

DETAILED DESCRIPTION

Disclosed herein are processes and systems that include active magneticregenerative refrigerators (AMRRs) for liquefying any process streamthat liquefies below ˜200 K including ethane, methane, argon, nitrogen,neon, hydrogen, or helium process gases. In certain embodiments, theprocess gas comprises hydrogen. In certain embodiments, the process gasconsists essentially of hydrogen (e.g., 95%, 96%, 97%, 98% or 99%hydrogen, with the remainder non-condensing or freezing impurities suchas helium gas). In certain embodiments, the process gas consists ofhydrogen.

The AMRR processes and systems can have several configurations such asreciprocating dual active magnetic regenerators or continuously rotatingwheel active magnetic regenerators with multiple layers of magneticrefrigerants that execute active magnetic regenerative cycles whencoupled to a reversing flow of heat transfer gas or liquid in themagnetized or demagnetized step of the cycle.

In particular, disclosed herein are configurations and processes inwhich an AMRR regenerator includes 2 to 16, more particularly eight,layers of compositionally-distinct magnetic refrigerant materials. Incertain embodiments, an AMRR apparatus is a dual regenerator apparatusthat includes a first AMRR regenerator and a second AMRR regenerator,wherein the first AMRR regenerator includes 2 to 16, more particularlyeight, layers of compositionally-distinct magnetic refrigerant materialsand the second AMRR regenerator includes 2 to 16, more particularlyeight, layers of compositionally-distinct magnetic refrigerantmaterials. In certain embodiments, the first AMRR regenerator and thesecond AMRR regenerator are structurally identical to each other. Whenin a top position (see, e.g. FIG. 1 for an example of “top”), themultiple layers of the AMRR regenerator are arranged sequentially fromhighest Curie temperature to lowest Curie temperature. The apparatus isconfigured so that the highest Curie temperature layer is contactedfirst by the heat transfer fluid in the demagnetized field, and lowestCurie temperature layer is contacted first by the heat transfer fluid inthe magnetized field. In other words, the layers are arranged insequential order from the highest Curie temperature layer to the lowestCurie temperature layer along the flow direction of the hot heattransfer fluid to cold heat transfer fluid, but are in sequential orderfrom the lowest Curie temperature layer to the highest Curie temperaturelayer along the flow direction when the flow is reversed from cold heattransfer fluid to hot heat transfer fluid.

Also disclosed herein are systems and processes that include multipleindependent AMRR stages, each AMRR stage including at least one AMRRregenerator having multiple layers of compositionally-distinct magneticrefrigerant materials. For example, in the case of hydrogen process gasa first stage with dual opposing identical regenerators, each with eightmagnetic refrigerant layers, cools the hydrogen process gas from 280 Kto 120 K, and a second stage with dual opposing identical regenerators,each with five magnetic refrigerant layers, cools the hydrogen processgas from 120 K to 20 K.

During a single AMR cycle all of the magnetic refrigerant materiallayers of the AMRR dual regenerators are simultaneously subjected to anequal high magnetic field in one step and all of the magneticrefrigerant material layers are simultaneously de-magnetized to a lowmagnetic field in another step. In other words, each individual layerduring the magnetization step is subject to the same magnetic fieldstrength at the same time, and each individual layer during thedemagnetization step is de-magnetized at the same time. These stepsoccur at different times in the reciprocating AMRR embodiment andsimultaneously in different regions (i.e., sections) of the wheel in arotary AMRR embodiment. In the reciprocating embodiment this isperiodically accomplished by axially moving the regenerators with themagnet stationary; in the rotary wheel embodiment this is continuouslyaccomplished by rotating the wheel through a high field region on about⅓ of the rim of the wheel and through a low field region oppositelylocated on about ⅓ of the rim of the wheel.

The layered active magnetic regenerators enable larger differencesbetween the average temperatures T_(HOT) and T_(COLD) necessary to usefewer stages in the new hydrogen liquefier design. The magneticregenerators are fabricated with multiple longitudinally orradially-layered magnetic refrigerants located such that the Curietemperature of each refrigerant is above the average AMR-cycle hottemperature T_(HOT) by ΔT_(HOT) at that axial location in theregenerators in steady-state operation to maximize refrigerant thermalmass differences and thereby percentage of bypass heat transfer gasflow. All the refrigerants in the AMRR individually execute smallmagnetic Brayton cycles as they are alternately magnetized anddemagnetized by the magnetic field and connected together from T_(HOT)to T_(COLD) by the flowing helium heat transfer gas. This couplingallows the overall temperature span of an AMRR to be many timesadiabatic temperature changes from the magnetocaloric effect of eachmagnetic refrigerant. The thermomagnetic properties of properly layeredrefrigerants must simultaneously have entropy flows that satisfy the 2ndlaw of thermodynamics with allowance for generation of irreversibleentropy and effects of bypass flows.

In certain embodiments, the multiple layers of different ferromagneticrefrigerants have Curie temperatures about 20 K (e.g., 18-22 K) apartbetween successively adjacent layers with the coldest layer adjacent tothe bypass flow outlet. In certain embodiments for hydrogen process gasat modest supply pressure such as ˜300 psia, the Curie temperature ofthe outermost layer to the Curie temperature of the innermost layer(i.e., the innermost layer is adjacent to the bypass flow outlet) spansfrom ˜293 K to ˜153 K. In certain embodiments for hydrogen process gas,the Curie temperature of the outermost layer to the Curie temperature ofthe innermost layer (i.e., the innermost layer is adjacent to the bypassflow outlet) spans from ˜293 K to ˜50 K. In certain embodiments formethane process gas at ˜200 psia, the Curie temperature of the outermostlayer to the Curie temperature of the innermost layer (i.e., theinnermost layer is adjacent to the bypass flow outlet) spans from ˜293 Kto ˜150 K. In certain embodiments for helium process gas at ˜15 psia,the Curie temperature of the outermost layer to the Curie temperature ofthe innermost layer (i.e., the innermost layer is adjacent to the bypassflow outlet) spans from ˜293 K to ˜18 K.

A further feature of the systems and processes disclosed herein is thediversion of a portion of the heat transfer fluid flow from each Curietemperature layer, except for the lowest Curie temperature layer or thelayer fluidly coupled to the bypass flow outlet, of the AMMR dualregenerators or AMRR apparatus in the demagnetized step (or regenerator)to the corresponding Curie temperature layer in the magnetized step (orregenerator). This controlled diverted flow allows each layer to havethe optimum mass of refrigerant, and the corresponding optimum flow ofheat transfer fluid (including bypass flows). In certain embodimentsdescribed herein, the systems and processes can provide refrigerationbetween 280 K and 120 K with an apparatus utilizing rotary wheels,rotary belts, or reciprocating regenerators with stationary magnets,each apparatus carrying regenerators comprised of layered ferromagneticmaterials with Curie temperatures between 293 K and 153 K. The processesand systems further utilize bypass flow of a portion of cooled heattransfer fluid (e.g., a gas) to pre-cool a separate process stream to beliquefied.

To make a highly efficient liquefier for hydrogen or other processgases, several features should be used in its design. These featuresinclude:

-   -   Use an inherently efficient thermodynamic cycle;    -   Use an efficient work input device or mechanism;    -   Use an efficient work recovery device or mechanism;    -   Insure small temperature approaches for heat transfer between or        among streams or between solids and streams;    -   Use high specific area and highly-effective regenerative and/or        recuperative heat exchangers;    -   Keep pressure drops for heat transfer gas flows and process gas        flow very low;    -   Invoke low longitudinal thermal conduction mechanisms via        material and geometry choices;    -   Minimize frictional and parasitic heat leak mechanisms; and    -   Specifically for hydrogen, perform ortho-to-para conversion at        the highest possible temperature during cooling in the process        heat exchangers.

In active magnetic regenerative liquefier (AMRL) designs rejection andabsorption of heat are achieved by the temperature increase or decreaseof magnetic refrigerants in regenerators upon isentropic magnetizationor demagnetization combined with reciprocating flow of heat transfergas. The cycle steps that magnetic refrigerants in an active magneticregenerator (AMR) execute are: i) magnetization with no heat transfergas flow; ii) cold-to-hot heat transfer gas flow at constant magnetichigh field; iii) demagnetization with no heat transfer gas flow; and iv)hot-to-cold heat transfer flow at constant low or zero field. The AMRcycle of one or more refrigerants thermally connected by heat transferfluid flow (e.g., a gas) in AMRR stages can be used to design excellentliquefiers whose potential for high performance comes from:

-   -   Almost reversible nature of magnetization-demagnetization steps        in an AMR cycle at up to hertz frequencies for certain magnetic        refrigerants. In contrast, it is inherently difficult to        reversibly achieve high compression ratios, high throughput, and        high efficiency in gas compression because of fundamentally poor        thermal conductivity of low-density gases such as hydrogen or        helium;    -   Efficient internal heat transfer between porous working        refrigerant solids and flowing heat transfer fluids (e.g., a        gas) in AMR cycles can maintain small temperature differences at        all times during the cycle by using geometries with high        specific areas such as ˜10,000 m²/m³ in high-performance        regenerators;    -   Efficient cooling of the hydrogen or other process gas and the        AMRR heat transfer gas. This is a critical element of efficient        liquefier design. To illustrate this point, the real work        required to operate a single AMRR (or any other type of        single-stage refrigerator between 280 K and 20 K) as a hydrogen        liquefier with the hydrogen process stream entering the AMRR at        280 K into a single process heat exchanger at will be at least 4        times larger than the ideal minimum work of hydrogen        liquefaction due to the large initial approach temperature in        the single process heat exchanger. The huge impact on FOM of        this single design feature illustrates the importance of        reduction of approach temperatures in process heat exchangers in        liquefiers of hydrogen. Conventional gas cycle liquefiers with        only two to four heat exchanger stages inherently limit their        FOM to about 0.50 before other real component inefficiencies are        incorporated. Reducing the approach temperatures in process heat        exchangers by using counterflowing bypass flow of a small        percentage of heat transfer gas is a unique feature of AMRL        designs to achieve a FOM greater than 0.5. Further the novel        processes and systems disclosed herein substantially reduce the        number of AMRR stages required for higher FOM over large        temperature ranges as explained more fully below;    -   High energy density from use of solid refrigerants in compact        regenerative beds can become high power densities with AMR        cycles at hertz frequencies; and    -   Safe, reliable, durable, compact, and cost-effective devices.

The above-explained desired features can be achieved by incorporatinginto the systems and processes at least one, and preferably acombination, of the following inventive aspects disclosed herein:

-   -   Continuous bypass flow to continuously pre-cool the process gas        stream. The bypass gas flow is determined by the amount        necessary to completely pre-cool the process gas stream while        maintaining small 1-2 K temperature approaches between        counterflowing bypass gas and process gas;    -   The heat capacity of the magnetic refrigerant changes with the        magnetic field, especially in the temperature region from the        Curie temperature to ˜25-30 K lower. Therefore, the thermal        mass, or heat capacity multiplied by the refrigerant mass, will        also vary. To take advantage of this phenomena unique to        ferromagnetic refrigerants in an AMR cycle, the mass flow rate        of heat transfer gas in the hot to cold flow region (low field)        of the AMR cycle must be several percent (e.g., 2-12%, more        particularly 2%, 3%, 4%, or 5%) larger than the mass flow rate        of heat transfer gas in the cold to hot flow region (high field)        of the same AMR cycle to balance (i.e., equivalent or close to        the same) the energy transfers in the low and high field regions        of the AMRR executing an efficient AMR cycle; the difference in        heat transfer gas flows is separated from the heat transfer flow        after the hot to cold flow in the AMRR to create a cold bypass        stream of heat transfer gas that is warmed as it is returned to        the hot temperature of the AMRR by flowing through the process        stream heat exchangers in the AMRR;    -   The temperature difference between the bypass heat transfer        entering the process heat exchanger at a first cold inlet        temperature and the process gas exiting the process heat        exchanger at a first cold exit temperature is 1 to 5 K, more        particularly 1 to 2 K;    -   The magnetic refrigerant operates at or below its Curie        temperature throughout an entire active magnetic regeneration        cycle because this is the region where the difference of thermal        mass between magnetized and demagnetized magnetic refrigerants        is maximized; and/or    -   The sensible heat of the process gas is entirely removed by the        bypass flow heat exchanger.

The embodiments of the novel processes and systems described hereinutilize the difference between thermal mass of each layer offerromagnetic refrigerants below their respective Curie temperatureswhen magnetized and demagnetized to enable use of bypass flow over alarge temperature span. The amount of bypass flow results from theadditional heat transfer gas required to change demagnetized refrigeranttemperatures through a regenerator in the hot-to-cold flow step of theAMR cycle over the heat transfer gas required to change magnetizedrefrigerant temperatures through its dual magnetized regenerator in thecold-to-hot flow step of the AMR cycle. The larger the difference inthermal mass, the larger the amount of bypass flow required in anoptimized AMRR. Because the thermal difference increases with differencein magnetic field during the AMR cycle, the highest practical magneticfield changes from high field regions at 6-8 T to low field regions at 0to 0.3 T, preferably a value such as, for example, 7 T to 0.3 T aredesired. Maximum utilization of this feature is accomplished byoperating each layered ferromagnetic refrigerant below its Curietemperature throughout its entire AMR cycle which requires maintainingthe average T_(HOT) of each layered refrigerant during its AMR cycle atleast ΔT_(HOT) below its respective Curie temperature. Further, becausethe magnetic field-dependent thermal mass difference also decreasesmonotonically as the temperature of each layered magnetic refrigerantdecreases below its respective Curie temperature in the regenerator, theaverage temperature difference between average T_(HOT) and averageT_(COLD) of each layer of magnetic material within a regenerator of anAMRR stage is chosen to be about 20 K. In certain embodiments, theoperating temperature span (average T_(HOT)−average T_(COLD)) of eachrefrigerant in the AMRRs is chosen to be ˜20-30 K to maximize differencein thermal mass of the layered ferromagnetic refrigerants to maximizethe possible bypass flow rate. Further, over the 20-30 K temperaturespan below the Curie temperature the adiabatic temperature change ofeach magnetic refrigerant for a given magnetic field change decreaseswith temperature to closely match the 2^(nd) law of thermodynamicsrequirements of highly efficient thermodynamic refrigeration cycles,i.e. ΔT_(COLD)=ΔT_(HOT)*T_(COLD)/T_(HOT).

The helium bypass gas flow rate for each AMRR stage is calculated fromcomplete enthalpy balance between that of the desired hydrogen or otherprocess gas flow rate for a particular AMRL liquefaction rate, e.g.kg/day, and the enthalpy of the helium bypass gas flow rate in thecounterflow bypass-gas to process-gas micro-channel or otherhighly-effective heat exchanger. This novel feature enables the sensibleand/or latent heats of the hydrogen or other process gas to becontinuously and entirely removed by the warming helium bypass gas withvery small approach temperatures in each process heat exchanger (PHEX)of AMRR stages of the liquefier. Use of this design feature applies nomatter what hydrogen or other process stream flow rate is desired, i.e.,the helium gas bypass flow rate is simply increased to completely coolthe hydrogen or other process stream to within 1-2 K of the coldesttemperature of the particular AMRR stage being considered. For example,the hydrogen process stream can be cooled to the bubble pointtemperature for a specified pressure (e.g., ˜20 K for LH₂ at 0.1013 MPa)by appropriate flow rate of bypass gas that is initially at ˜18 K beforeit warms to ˜278 K in the continuous process heat exchanger. The averageT_(COLD) of the coldest AMRR stage in a hydrogen AMRL will be 1-2 Kbelow the LH₂ temperature. The variable in this design procedure is thehelium bypass flow; as it increases with increasing LH₂ liquefaction,the helium heat transfer flow through the magnetic regeneratorsincreases which in turn increases with the refrigeration capacity of theAMRR stages.

With this technique, the helium bypass flow rate is determined by theAMRL hydrogen or other process gas liquefaction rate. In turn, thehelium bypass gas flow rate is an optimum small fraction of the heliumheat transfer gas flow rate for the coldest layer of the regenerators inthe AMRR stage. The total heat transfer gas flow rate for each layer isdirectly coupled to the detailed design variables of the AMRR includingthe number and mass of each layer of magnetic refrigerants, adiabatictemperature changes, magnetic field change, heat capacity of therefrigerants, the cycle frequency, and temperature profile from T_(HOT)to T_(COLD) within each layer of the multi-layered regenerator thatdetermines the fraction of the active magnetic regenerator that coolsbelow the average T_(COLD) each cycle. With an optimum hot-to-cold flowrate of helium heat transfer gas through the active magneticregenerators the average cooling power of each layer of the demagnetizedregenerator (using Gd and several Gd-RE alloys as examples of anexcellent magnetic refrigerants from ˜280 K to ˜120 K) is given by thefollowing equation for Gd as: {dot over(Q)}_(Gd)(T_(COLD))=vFr_(COLD)M_(Gd)C_(Gd)(T_(COLD),B_(0.6T))ΔT_(CD)(T_(COLD))where {dot over (Q)}_(Gd)(T_(COLD)) is the cooling power in W, v is theAMR cycle frequency in Hz, Fr_(COLD) is a blow-averaged dimensionlessfraction of the regenerator that is colder than the average T_(COLD) ofthe demagnetized regenerator before the hot-to-cold blow of the heliumheat transfer gas, M_(Gd) is the mass of Gd in a flow sector of therotary wheel regenerator in kg, C_(Gd) is the total heat capacity of Gdat T_(COLD) and the low magnetic field after demagnetization in J/kg K,ΔT_(CD) is the adiabatic temperature change upon demagnetization fromhigh field to low field in K. All the variables in this equation areknown except Fr_(COLD). This parameter depends on the axial temperatureprofile of each layer of a multi-layer AMR that depends upon the coldthermal load for each layer, the heat transfer gas flow rate for eachlayer, and the percentage of bypass flow for each layer; a typical valueis ˜0.3. This equation applies to each of the magnetic refrigerants in alayered regenerator. Numerical simulation of axial temperature profilesfor active magnetic regenerators comprised of one or multiple layers ofrefrigerants as a function of total helium heat transfer gas flow andpercentage of bypass flow of heat transfer gas can be predicted bysolving the partial differential equations that describe AMRperformance. A linear axial or radial temperature profile in each layerof magnetic refrigerant in the subject invention is a good approximationfor optimum helium heat transfer gas and percentage bypass flow andprediction of Fr_(COLD). Experiments confirm the numerical predictionsthat 3-12% bypass is the typical range for each layer depending upon themagnetic refrigerant and magnetic field changes. The helium heattransfer gas flow rate for each layer is given by dividing the coolingpower of the regenerator by the heat capacity of helium at constantpressure times ΔT_(CD)/2 in kg/s. In the multilayer regeneratordescribed herein the heat transfer gas flow rate is largest in theoutermost hotter layer, and smallest in the innermost colder layer andto achieve the optimal heat transfer gas flow rate of each layer, theflow of heat transfer gas is adjusted by diverting a portion of the heattransfer gas flow for sequential layers in the demagnetized layeredregenerators to the corresponding magnetized layered regenerators toprovide a reduced flow for the next colder layer in the multi-layerregenerator in the hot to cold flow region and an increased flow in thenext warmer layer in the multi-layer regenerator in the cold to hot flowregion. This is illustrated in FIG. 2.

Certain embodiments of the novel processes and systems have bypass flowof a few percent of cold helium gas (e.g., 3 to 12% of the total heattransfer gas in the hot-to-cold flow through the demagnetizedregenerator, more particularly 6%) to continuously pre-cool the hydrogenor other process gas stream thus reducing the number of AMRR stages inan efficient H₂ liquefier from 6-8 stages without bypass flow to 2-3stages with bypass flow. It is obvious fewer AMRR stages using bypassflow to continuously pre-cool the hydrogen process stream will alsorequire substantially less magnetic refrigerant for an equivalentliquefaction rate. Continuous cooling of the hydrogen process stream ina liquefier essentially eliminates a very large source of irreversibleentropy generation caused by larger approach temperatures in the processstream heat exchangers and thereby increases the FOM of the liquefiersignificantly. From this description it is apparent that the flow rateof bypass heat transfer gas is determined by complete removal of thesensible and latent heats from the process stream which in turndetermines the total helium heat transfer flow rate for the AMRR stagegiven the percentage of bypass flow allowed by the thermal massdifferences in the magnetic regenerator which in turn determines themass of the magnetic regenerator given the ΔT_(COLD) from themagnetocaloric effect. This novel design process minimizes the number ofAMRR stages and mass of magnetic refrigerants in liquefiers for hydrogenand other gases. For example, in certain embodiments the process andsystem has only two stages; a first stage for cooling the process gasfrom 280 K to 120 K and a second stage for cooling the process gas from120 K to 20 K or 4 K.

The magnetic refrigerants in the AMR beds have a difference in thermalmass which is the product of heat capacity per unit mass times the massof magnetic refrigerant (or just heat capacity in this case because themass of magnetic material in a magnetic regenerator doesn't depend upontemperature or magnetic field). The heat capacity of a ferromagneticmaterial below the Curie temperature (ordering temperature) is smallerin higher magnetic fields than at lower or zero magnetic fields.However, this difference switches at the Curie temperature because theheat capacity in higher magnetic fields decreases slowly as thetemperature increases while the heat capacity in low or zero fieldsdrops sharply at the Curie temperature such that the heat capacity athigher fields becomes larger than the heat capacity in lower or zeromagnetic fields. This means the difference in thermal mass between amagnetized AMR bed and a demagnetized AMR bed changes sign at the Curietemperature and the net difference in thermal mass for an AMR cyclespanning across the Curie temperature rapidly decreases with increasingtemperature. Therefore, the optimal amount of bypass flow for an AMRcycle that extends above the Curie temperature will rapidly decrease tozero. Simultaneously an AMR cycle operated this way will become lessefficient due to increased intrinsic entropy generation in such an AMRcycle and due to insufficient bypass flow to pre-cool the same amount ofhydrogen process gas. In the design of the novel processes and systemsdisclosed the importance of selecting and controlling the hot sinktemperature and temperature span to maximize the difference in thermalmass (and thereby the amounts of bypass flow) is recognized. First, thedynamic T_(HOT) is always ΔT_(HOT) less than the Curie temperature ofthe magnetic refrigerant at the hot end of the regenerator (i.e., theouter-most refrigerant in the layered rim of the wheel in FIG. 1) at itsmaximum during the magnetization step of the AMR cycle. T_(HOT) is theenvironmental temperature where the heat is dumped. The dynamic T_(HOT)is the increase in temperature caused by inserting the regenerator intothe magnetic field. The maximum dynamic T_(HOT) depends on where it isin the cycle, but generally the maximum is T_(HOT)+ΔT_(HOT). This can bedone by setting a fixed heat sink temperature to anchor T_(H) which inturn yields the largest difference in thermal mass between high and lowmagnetic fields. The second aspect of the difference in thermal mass inhigh and low magnetic fields is that it decreases steadily as the coldtemperatures in the regenerator decrease below the Curie temperature (s)of the magnetic refrigerants. Hence, the magnetic materials in an AMRbed must operate in temperature spans when magnetized ofT_(H)+ΔT_(HOT)<T_(Curie) and T_(C)+ΔT_(COLD) equal to ˜20 K<T_(Curie)and when demagnetized, between T_(H)−ΔT_(HOT) and T_(C)−ΔT_(COLD) whichare ˜20 K apart. T_(C) represents cold temperatures of a slice ofmagnetic regenerator at any point in the AMR as it executes its tinymagnetic Brayton cycle. ΔT_(COLD) represents the temperature drop causedby the magnetocaloric effect when the regenerator is removed from themagnetic field. If larger temperature spans with optimum differences inthermal mass are desired (as required for very high FOM), layers ofmagnetic materials with descending Curie temperatures must be used inthe AMR bed.

An illustrative embodiment of an AMRR apparatus is shown in FIG. 1. TheAMRR apparatus includes an 8-magnetic refrigerant material layer dualAMR regenerator arrangement—first AMR multilayer regenerator 1 andsecond AMR multilayer regenerator 2. This AMRR apparatus is suitable foruse in a reciprocating regenerator configuration with a stationarymagnet. A similar arrangement exists in a rotary wheel as shown in FIG.5, described below in more detail, where the section of the layeredwheel rim in the high field region of the wheel is a first AMRregenerator segments and the section of the same wheel rim in the lowfield region of the wheel is a set of second AMR regenerator segments.The individual layers in the first AMR regenerator 1 are labeled 1 t, 2t, 3 t, 4 t, 5 t, 6 t, 7 t and 8 t. The corresponding individual layersin the second AMR regenerator 2 are labeled 1 b, 2 b, 3 b, 4 b, 5 b, 6b, 7 b and 8 b. A first diversion fluid conduit 3 fluidly couples a heattransfer fluid outlet of layer 1 t with a heat transfer fluid inletlayer 1 b. Similarly, a second diversion fluid conduit 4 fluidly couplesa heat transfer fluid outlet of layer 2 t with a heat transfer fluidinlet layer 2 b, a third diversion fluid conduit 5 fluidly couples aheat transfer fluid outlet of layer 3 t with a heat transfer fluid inletlayer 3 b, a fourth diversion fluid conduit 6 fluidly couples a heattransfer fluid outlet of layer 4 t with a heat transfer fluid inletlayer 4 b, a fifth diversion fluid conduit 7 fluidly couples a heattransfer fluid outlet of layer 5 t with a heat transfer fluid inletlayer 5 b, a sixth diversion fluid conduit 8 fluidly couples a heattransfer fluid outlet of layer 6 t with a heat transfer fluid inletlayer 6 b, and a seventh diversion fluid conduit 9 fluidly couples aheat transfer fluid outlet of layer 7 t with a heat transfer fluid inletlayer 7 b. The amount of heat transfer fluid diverted through thediversion fluid conduits may vary, but in certain embodiments, thevolume percentage of total flow for diverted fluid per each layerincreases from 18% to 59%, or 15% to 60%, from layer 1 to layer 7 withzero diverted flow in the 8^(th) layer of the heat transfer fluid flowvolume flowing into the inlet of the respective layer. In certainembodiments, the individual diversion flows are diverted at a locationbetween adjacent layers in the demagnetized region and re-injected at alocation between corresponding adjacent layers in the magnetized region.The diverted flows for a reciprocating dual regenerator configurationare illustrated in FIG. 1.

In a rotary wheel configuration the dual regenerators are on oppositesections of the wheel and the diverted flow channels, although not shownin FIG. 5, are placed circumferentially around the wheel rim such thatdiverted flow from thin spacers between the layers of the demagnetizedset of regenerator segments with hot-to-cold heat transfer gas flow tothe same thin spacers in the corresponding layers of the magnetized setof regenerators with cold to hot heat transfer gas flow. The amount ofdiverted flow is controlled by controlled diversion flow valves for eachdiversion flow channel for each corresponding layer. A controllablediversion flow valve may be present between adjacent magnetized layersthat reduces the flow of heat transfer fluid into the next smaller layerand transfers the diverted flow into the corresponding layers in themagnetized dual regenerator. During cool down of the dual regeneratorsfrom an initial temperature of ˜280 K in all layers, independent controlof each diversion-flow valve allows the sequential cool down of thelayers as they are cooled several degrees below their respective Curietemperatures. After all layers are in their respective operatingtemperature ranges, the diversion flows can be tuned to optimize coolingperformance of the dual regenerator. A multilayer AMRR can be cooleddown by controlling the diversion flow valves to initially flow onlythrough a top most layer that is below its Curie temperature at 280 Kuntil the next adjacent layers are cooled to below their respectiveCurie temperatures to enable normal steady-state operation.

During operation of the AMRR apparatus each layer of magneticrefrigerant in a multilayer regenerator has a different heat transfergas flow rate relative to each adjacent layer. The difference in heattransfer gas flow rate between one layer and the next adjacent layer mayrange proportionally to provide optimum heat transfer fluid flow ratesin each sequentially smaller layer as shown in FIG. 3. For example, thedifference in heat transfer gas flow rate between one layer and the nextadjacent layer may range from 27 percent between the first and secondlayers to 1.5 percent between the seventh and eight layers.

A non-diverted or main stream 16 of the heat transfer fluid flows fromeach layer to the adjacent layer. Each separate gas stream 16 betweenadjacent layers is about 20 K different in temperature as expectedexcept the coldest layer(s) that provides the single bypass stream tocontinuously cool the process stream and then supplies the remainingportion or unbalanced portion as a cold to hot flow 17 for themagnetized layer(s), starting with the coldest layer 8 b. The bypassflow portion constitutes 1 to 15, more particularly 4 to 6, weightpercent of the total amount of heat transfer fluid exiting the coldestlayer of low magnetic or demagnetized field section.

An outlet of the coldest Curie temperature refrigerant layer (layer 8 t)is fluidly coupled to a bypass flow conduit 10 (in the embodiment shownin FIG. 1 there is only a single bypass flow conduit). The bypass heattransfer fluid flow is sent to a bypass heat exchanger 11 from the coldheat transfer fluid flowing out of the cold duct in the low field ordemagnetized region to maintain steady-state flow and thus the verysmall 1 to 5 K, more particularly 0.5-2 K, approach temperatures betweenbypass flow and counterflowing process gas at all times and locations inthe bypass heat exchanger 11. In the rotary wheel configuration thebypass heat transfer fluid flow is continuously sent to the bypass heatexchanger. In the reciprocating dual regenerator configuration, acontinuous bypass flow may be accomplished with two identical dualregenerator systems out of phase with each other by 90 degrees so oneregenerator of the four in this embodiment provides cooling with severalthree-way valves to provide continuous bypass flow into the process heatexchanger during the appropriate hot to cold flow periods of the AMRcycle in the reciprocating configuration.

The bypass heat transfer fluid is introduced into the bypass heatexchanger 11. The bypass heat transfer fluid cools the process gas thatis also introduced into the bypass exchanger 11. The process gas isdelivered to the bypass heat exchanger 11 via a process gas conduit 12that is fluidly coupled to a process gas source. If the process gas ishydrogen, the source may be an electrolyzer, an autoreformer,steam-methane reformer or another source. In certain embodiments, forhydrogen liquefaction, the process gas channels of the bypass heatexchanger include at least one ortho H₂ to para H₂ catalyst. Theexothermal catalysis heat and sensible heat in the hydrogen gas streamare removed only via the bypass flow in the process heat exchanger.

In certain embodiments, the exhaust heat of the AMRL from the 1^(st)layer of the dual layer configuration in FIG. 1 is rejected into a hotheat sink exchanger by the heat transfer gas in the cold to hot flow.The hot heat sink exchanger is cooled by a fluid flow such aswater-glycol mixture from the controllable hot heat sinker chiller. Thischiller also cools the process stream gas to ˜280 K before it enters theprocess stream heat exchanger.

The bypass heat transfer fluid exiting the bypass heat exchanger iscombined with a hot heat transfer fluid flow exiting the high magneticfield section (i.e., layer 1 b). In certain embodiments, the bypass heattransfer fluid exiting the bypass heat exchanger is combined with thehot heat transfer fluid flow at the suction side of a pump thatcirculates the heat transfer gas. The combined bypass heat exchangerexit fluid flow and hot heat transfer flow is introduced into thehighest Curie temperature layer (i.e. layer 1 t) via introductionconduit 13. The hot heat transfer flow is fluidly coupled to the bypassheat exchanger exit fluid flow via conduit 15. In certain embodiments,the mixed bypass heat exchanger exit fluid flow and hot heat transferflow may pass through an optional chiller hot heat sink exchanger 14.The exhaust heat from a thermodynamic cycle such as the AMRR must beremoved to complete the cycle and the temperature-controlled chiller isthe means to do this. It also allows setting of the steady-state T_(HOT)of the AMRR at ˜280 K.

FIG. 1 shows a dual regenerator AMRR that includes a first regenerator 1and a second regenerator 2. The reciprocating embodiment shown in FIG. 1depicts the upper or top regenerator 1 in a demagnetized state and thelower or bottom regenerator 2 in a magnetized state. Of course, sincethe apparatus reciprocates in and out of a magnetic field, thedemagnetized/magnetized state of the respective regenerators 1 and 2will continuously reverse from each other.

In the rotary wheel configuration the wheel rotation continually causeslayered regenerator segments in the rim of the wheel to be entering thehigh field region simultaneously as other identical layered regeneratorsegments are entering the low field region of the wheel. An illustrativerotary wheel embodiment is shown in FIG. 5.

The rotary AMRR apparatus of FIG. 5 includes an annular bed 51 of atleast one porous magnetic refrigerant material. As shown in FIG. 5, therotary AMRR apparatus is divided into four sections (listed in order ofwheel rotation): (i) a high magnetic field section in which the heattransfer gas flows from a cold side to a hot side through the magnetizedbed(s), (ii) a first no heat transfer gas flow section in which thebed(s) are demagnetized, (iii) a low magnetic or demagnetized fieldsection in which the heat transfer gas flows from a hot side to a coldside through the demagnetized bed(s), and (iv) a second no heat transfergas flow section in which the bed(s) are magnetized. Circumferentialseals are provided in the no circumferential heat transfer gas flowsections to prevent the heat transfer gas flow. Radial seals areprovided in the radial heat transfer gas flow sections to prevent theheat transfer gas flow over or under regenerators so only radial flowoccurs. The multilayer magnetic regenerators may be divided intocompartments 56 wherein the compartments eliminates circumferential flowof heat transfer gas through the porous regenerators while allowingradial flow of heat transfer gas.

The rotary AMRR apparatus includes a rotating wheel that includes aninside hollow annular rim 52 (inner housing and flow duct wall) and anoutside hollow annular rim 53 (outer housing and flow duct wall). A hotheat transfer fluid (HTF) (e.g., helium gas) is introduced into theoutside rim 53 of the rotary AMRR apparatus via an HTF inlet ductprovided in the low magnetic or demagnetized field section (iii). Thehot HTF in the outside rim 53 has a steady-state circumferentiallyaverage temperature that, for example, may be 280-285 K. However, thelocal temperature at a given time and location in the AMR cycle maydiffer from the steady-state circumferentially average temperature. Thehot HTF flows in a radial direction through the low magnetic ordemagnetized bed, cooling the HTF. The cooled heat transfer fluid exitsthe low magnetic or demagnetized field section (iii) via an HTF outletduct and into the inside rim 52. The HTF radial flow is shown by thearrows 54 in the low magnetic or demagnetized field section (iii). Thecold HTF in the inside rim 52 has a steady-state circumferentiallyaverage temperature that, for example, may be 125-130 K or 118-123 K.However, the local temperature at a given time and location in the AMRcycle may differ from the steady-state circumferentially averagetemperature. The inside rim 52 is fluidly coupled via an HTF outlet ductand a conduit to an inlet of an optional cold heat exchanger (CHEX). Ifdesired, the CHEX is for the reject heat from a colder AMRR stage andfor very small parasitic heat leaks. As can be seen the approachtemperature differential in the CHEX is 1 K.

The heat transfer fluid exits the CHEX and into a T-junction in which aportion of the heat transfer fluid bypasses the high magnetic fieldsection (i) and instead is directed to an inlet of a bypass gas heatexchanger. The flow at the T-junction may be controlled a bypass flowcontrol valve. In certain embodiments, 3-12%, particularly less than12%, more particularly less than 8%, and most particularly 6%, of theheat transfer fluid is diverted to the bypass gas heat exchanger. Theremaining heat transfer fluid is introduced as the cold flow into theinside rim 52 at the high magnetic field section (i) via an HTF inletduct.

The cold HTF flows in a radial direction through the high magnetizedbed, heating the HTF. The hot HTF exits the high magnetic field section(i) via an HTF outlet duct and into the outside rim 53. The HTF radialflow is shown by the arrows 55 in the high magnetic field section (i).The hot HTF exits the high magnetic field section (i) and is introducedvia a conduit to into a hot heat exchanger (HHEX). The HHEX cools theheat transfer fluid down to a suitable temperature for introduction asthe hot flow into the low magnetic or demagnetized field section (iii).

As mentioned above, the bypass HTF is introduced into a bypass HEX. Thebypass HTF cools the process gas that is also introduced into the bypassHEX. In certain embodiments, the bypass HEX includes at least one orthoH₂ to para H₂ catalyst. The heat from the exothermic catalysis andsensible heat in the cooling process gas stream are removed only via thebypass HEX. In other words, no other heat exchangers are required toremove the catalytic and sensible heat (as mentioned above, the CHEXonly removes the reject heat from a colder AMRR stage and very smallparasitic heat leaks). As can be seen the approach temperaturedifferential in the bypass HEX is 1K.

The bypass HTF exiting the bypass HEX is mixed with the hot HTF flowexiting the high magnetic field section (i). The mixed bypass HEX andhot HTT flow is introduced into the HHEX.

For optimal heat transfer, different mass flow rates of heat transfergas are required in these two stages (i.e., magnetized vs.demagnetized), and this is accomplished by bypass of some cold heattransfer gas from the hot-to-cold flow step before the cold-to-hot flowstep of the cycle. For example, maximum use of continuous flow of coldsensible heat in the bypass stream as it returns to higher temperaturesin a counterflow heat exchanger to continuously cool a process gasstream can increase the FOM of an active magnetic regenerative liquefier(AMRL) from ˜0.35 in conventional gas-cycle liquefiers to ˜0.60 or morein AMRLs. Besides increasing the FOM, the use of bypass stream tocontinuously and completely cool the process gas significantly reducesthe refrigeration cooling capacity per AMRR stage and thereby reducesthe mass of magnetic refrigerants required in the AMRL. Rotary AMRLsintrinsically have continuous bypass gas flow for continuous pre-coolingof a process gas stream while reciprocating AMRLs need at least two setsof dual regenerators with proper phasing in/out of the magnetic fieldwith three-way valves to provide continuous bypass gas flow into theprocess heat exchangers.

In certain embodiments, the diameter of an assembled dual regeneratorsubsystem has to fit inside the high-field volume inside of asuperconducting magnet such that either one of the dual regenerators issequentially in the high-field region such as 6-7 T. The separationbetween the centers of the dual layered regenerators should besufficient to ensure a driver such as a linear actuator will be able toaxially move enough to ensure either regenerator is in the low fieldsuch as ˜0.3 T in the demagnetized step during the AMR cycle. In FIG. 1this axial distance is the distance from the spacer between the4^(th)-5^(th) layers of the upper regenerator to the corresponding4^(th)-5^(th) layers of the bottom regenerator. In the rotary wheelconfiguration this distance is determined by selecting the wheeldiameter and the design of the superconducting magnet such that the highfield region of the wheel is in 6-7 T and the low-field region of thewheel is in ˜0.3 T. The heat transfer fluid mass flow is directlyproportional to the process gas flow rate, the percent bypass flow, andthe amount of magnetic refrigerant per layer, so to fully utilize eachlayer of magnetic refrigerant, it is important to adjust the heattransfer gas mass flow between each layer.

For example, given a GH₂ mass flow rate of 2.89×10⁻⁵ kg/s or equivalentprovides ˜10 gallons/day of LH₂/day. The total sensible heat thermalload including ortho-para conversion to cool this flow rate ofequilibrium/normal GH₂ from 280 K to equilibrium GH₂ at 120 K is 62.4 W.Assuming the cold helium bypass flow at 200 psia enters the process heatexchanger at 118 K and warms to 278 K while providing continuous coolingof the GH₂, the mass of bypass helium is 7.50×10⁻⁵ kg/s.

The hot-to-cold helium mass flow enters the demagnetized 8^(th) layer at140 K and exits this layer at 118 K and with 6% bypass, the resultanthot-to-cold helium mass flow rate in the demagnetized 8^(th) layer is0.72×10⁻³ kg/s up to 1.25×10⁻³ kg/s. The difference in helium mass flowinto layer 1 t and into layer 8 t of the demagnetized regenerator can beaccomplished by diverting a controlled amount of helium flow (afterlayer 1 t and before layer 2 t) in the demagnetized regenerator andre-introducing the diverted flow at the corresponding location (betweenlayer 2 b and layer 1 b) in the magnetized regenerator and similarly forthe other layers in the dual regenerators.

The results of our design calculations for an illustrative flow throughan 8-layer dual regenerators are summarized in Table 1 below.

Mass of magnetic Work Average Curie material/ rate/ Q_(HOT)/T_(HOT)/T_(COLD) Temp layer layer layer Layer Material (K) (K) (grams)(W) (W) 1 Gd 280/260 293 268 11.0 132 2 Gd_(0.9)Y_(0.1) 260/240 274 2589.9 110 3 Gd_(0.3)Tb_(0.7) 240/220 253 235 8.8 90.7 4 Gd_(0.69)Er_(0.31)220/200 232 202 7.6 71.9 5 Gd_(0.32)Dy_(0.68) 200/180 213 172 6.3 54.4 6Gd_(0.15)Dy_(0.85) 180/160 193 139 4.9 38.4 7 Gd_(0.27)Ho_(0.73) 160/140173 100 3.4 23.9 8 Gd_(0.16)Ho_(0.84) 140/120 153 57 1.8 11.0The ideal refrigeration power required to cool this flow rate of 300psia GH₂ stream from 280 K to 120 K is 28.7 W. With the existing 8-layerdual regenerators and the work rates and modest regenerator efficienciesfor each layer, the initial calculated relative efficiency is 53.5%.

A number of factors can be taken into account for designing an AMRRsystem. The regenerators needed to fit within a magnet bore, the spacingbetween the regenerators may be limited by the actuator stroke length,and in order to limit the amount of parasitic heat leak it was desirableto have the exterior of the regenerators in vacuum. The high-fieldsuperconducting magnets are also in vacuum and separately conductivelycooled to ˜4 K by a cryocooler (not shown).

An illustrative design includes two regenerators separated by a centersection where the bypass is pulled from, and all three of these sectionsare held together with a compressive bolt load. For example, FIG. 2shows a cross-section of a first regenerator 20 and a second regenerator21. A center section 22 is disposed between the first regenerator 20 andthe second regenerator 21. An external vacuum tube 23 encompasses theregenerators 20 and 21 and the center section 22 enabling establishing avacuum around the exterior of the regenerators to reduce parasitic heatleaks.

FIG. 3 shows a cross-section of one of the dual regenerators. Themagnetic refrigerant layers are colored sections in FIG. 3 and representeach of the eight layers 30, 31, 32, 33, 34, 35, 36, and 37 indescending order from highest Curie temperature layer 30 to lowest Curietemperature layer 37. The mass of each of the eight layers 30-37 differfrom each other in descending order from highest Curie temperature layer30 to lowest Curie temperature layer 37. In certain embodiments, themass difference between adjacent layers is 1 to 50, more particularly 4to 43 percent. The blue colored layer 37 is the coldest layer with thesmallest diameter (e.g., 1.25″) and the lowest mass. The layers get0.25″ larger in diameter (e.g., 0.25″ increase per layer) as they moveup and the mass increases up to the red layer 37 that is the warmestwith the largest diameter and mass. The diameter, mass and thickness ofeach layer is listed in Table 2. The layers 30-37 are each separatedfrom each other by a mesh layer 38 (depicted as a green colored layer)that allows a main heat transfer fluid 39 to flow in the direction of alongitudinal axis 40 of the regenerator and allows a portion of the heattransfer fluid to flow out radially to diversion flow channels 41 ateach layer. In one embodiment, the combined height of the layers is 4.77inches, but the regenerator is actually taller due to the mesh layer.The final total height of each regenerator is 5.2″ which is near themaximum height a regenerator could be for an 8″ tall magnet. Theregenerator may be built from a series of machined parts and adheredtogether. The diversion flow channels 41 are incorporated into each ofthe machined parts and are formed when the parts are sealed together.This eliminates most of the external plumbing and eases assemble, andalso should minimize the heat leak. The warmest diversion flow channelis disposed closest to the outside of the regenerator and eachsubsequent diversion channel moves inward toward the center of theregenerator. This effectively allows the diversion flow of one layer toprovide a cooling ring around the layer below it. The diversion flow iscollected into a single tube at the bottom of the regenerator where itflows into the center section and is sealed to the middle section by acompressive Teflon seal. FIG. 2 shows a centrally aligned conduit 42 forthe main heat transfer fluid flow, and a peripherally located conduit 43for the individual diversion flows. The amount of pressure drop andtherefore the amount of diversion flow can be changed by changing theamount or size of flow impedance beads in the tubes of the centersection. The diversion flow for each layer will have its own tube in thecenter section where the pressure drop can be adjusted.

In FIG. 4 there are four diversion flow channels per each layer exceptthe 8^(th) layer which only has bypass flow and no diversion flow. Fourdiversion flow channel openings between each layer are machined into thecontainer for the layered regenerator. Four thin-wall stainless steeltubes are epoxy sealed to the openings between each layer to connect tothe corresponding opening in the same location in the identical dualregenerator. The diversion flow is adjusted by controlled flow impedanceof the tubes.

TABLE 2 The physical properties and flow rates for each layer in theregenerators. Pressure Drop for Mass Thickness Diameter He Flow DPDiversion Diversion Layer (g) (″) (″) (L/s) (psia) Flow (L/s) Flow(psia) Layer 8 57 0.561 1.25 0.133 0.213 N/A N/A [coldest stage] Layer 7100 0.683 1.5 0.32 0.493 0.187 0.426 Layer 6 139 0.698 1.75 0.55 0.6970.23 1.412 Layer 5 172 0.661 2 0.83 0.805 0.28 2.806 Layer 4 202 0.6142.25 1.16 0.86 0.33 4.416 Layer 3 235 0.578 2.5 1.58 0.934 0.42 6.136Layer 2 258 0.525 2.75 2.02 0.931 0.44 8.004 Layer 1 268 0.458 3 2.460.855 0.44 9.866 [warmest stage] Total 1431 4.777 9.053 5.788Table 2 shows the flow rate needed for each layer and the anticipatedpressure drop for that flow in this particular example. It also showsthe diversion flow rate and pressure drop needed in the diversion flowchannel to obtain the desired flow. The diversion flow needed for eachlayer is the difference in the main He flow between the layer and thecooler layer below it. The pressure drop needed for the diversion flowis the sum of all the pressure drops of the layers between where thediversion flow separates from the main flow to where it joins back upwith it. So for example, the diversion flow channel for layer 6 has tomatch the flow impedance of the combined pressure drops of layers 7 and8 of both regenerators.

In the embodiment shown in FIGS. 2 and 3, the main fluid flow connectionto the lower regenerator is routed through channels built into the outeredge of each regenerator. These channels then connect to one of fourtubes running through the center section. Four tubes are required due tothe higher gas flow rate. The four tubes of the center section thenconnect to the channels on the other regenerator and finally combineback to one flow at the end of the lower regenerator.

FIG. 4 shows a CAD model of the flow channels for the main flow beingrouted around the regenerators. In the designs shown in FIGS. 2-4,diversion flow channels are provided around the circumference of eachlayer of magnetic refrigerant.

Good magnetic refrigerants have large magnetic moments to providemaximum entropy change from changes in magnetic field. The accompanyingmagnetocaloric effect of a good material is confined to a finitetemperature range around its magnetic ordering temperature where themagnetic entropy is strongly temperature and field dependent. To takemaximum advantage of bypass flow it is important to maximize thedifference between the high-field and low-field thermal mass of magneticrefrigerants. The thermomagnetic properties of the refrigerants mustsimultaneously satisfy numerous other criteria such as: i) satisfyingthe adiabatic temperature changes as a function of temperature tosatisfy the 2nd law of thermodynamics and ii) allowance for inevitablecreation of some irreversible entropy even in the best optimizedregenerator designs.

Gadolinium is an excellent magnetic refrigerant and has been generallyaccepted as the reference material against which other refrigerants arecompared. It has a simple ferromagnetic ordering temperature of ˜293 Kand exhibits an adiabatic temperature change of ˜2 K/Tesla overpractical magnetic field strengths (up to ˜8 T). It also has a largedifference in field-dependent thermal mass just below its Curietemperature. Introduction of alloying additions of another lanthanidemetal reduces the magnetic-ordering temperature of Gd without mucheffect on the total magnetic moment per unit volume and the change inmagnetization with temperature near a sharp ordering temperature.

Homogeneous alloys of Gd with other rare earth metals (Tb, Er, Dy, Ho)or Y make superior magnetic refrigerants as well. Other potential rareearth elemental refrigerants such as Ho and Er have more complexmagnetic ordering phenomenon but when alloyed with Gd these effects tendto be reduced at high magnetic fields. The addition of non-magnetic Y toGd reduces the adiabatic temperature change of Gd gradually butsimultaneously decreases the magnetic ordering temperature so the simpleferromagnetism of Gd is preserved down to about 200 K.

Key features or suitable refrigerant materials include:

-   -   Use ferromagnetic materials that operate below their Curie        temperature throughout their entire AMR cycle;    -   Maintain average T_(HOT) at least ΔT_(HOT) below the Curie        temperature of the uppermost layer of magnetic material in a        regenerator; this applies to each layer of magnetic material in        the regenerator with correspondingly lower cycle temperatures;    -   Average temperature difference between T_(HOT) and T_(COLD)        should be ˜20 K per layer of magnetic refrigerant;    -   Spanning from 280 K to 120 K in one AMRR stage requires 8        refrigerants to be combined into optimally layered regenerators.    -   Layering must have smooth flows of energy and entropy at        transitions between layered refrigerants along the longitudinal        axis of the regenerator.

Illustrative magnetic refrigerants include those shown below in Table 3.

Operating Temperature Span Ordering Temperature Material K K Gd 280-260293 Gd_(0.90)Y_(0.10) 260-240 274 Gd_(0.30)Tb_(0.70) 240-220 253Gd_(0.69)Er_(0.31) 220-200 232 Gd_(0.02)Tb_(0.98) 220-200 233Gd_(0.32)Dy_(0.68) 200-180 213 Gd_(0.66)Y_(0.34) 200-180 213Gd_(0.39)Ho_(0.61) 180-160 193 Gd_(0.59)Y_(0.41) 180-160 193Gd_(0.15)Dy_(0.85) 180-160 193 Gd_(0.42)Er_(0.58) 160-140 173Gd_(0.27)Ho_(0.73) 160-140 173 Gd_(0.16)Ho_(0.84) 140-120 153Gd_(0.34)Er_(0.66) 140-120 152 Gd_(0.23)Er_(0.77) 120-100 132(Ho_(0.80)Gd_(0.20))Co₂ 120-100 130 Ho_(0.90)Gd_(0.10)Co₂ 100-80  110Ho_(0.95)Gd_(0.05)Co₂ 80-60 90 Gd_(0.5)Dy_(0.5)Ni₂ 60-40 70Dy_(0.75)Er_(0.25)Al₂ 40-20 50

Another illustrative refrigerant material is Gd_(x)Er_(1-x)Al₂; Curietemperatures range from 168 K if x=1 to 15 K if x=0 for operationbetween ˜150 K and ˜20 K.

Illustrative ortho H₂ to para H₂ catalysts for use in the bypass flowprocess heat exchangers include, but are not limited to, activatedcarbon; ferric oxide (Fe₂O₃); chromic oxides (Cr₂O₃ or CrO₃); Ni metaland Ni compounds (Ni²±); rare earth metals and oxides such as Gd₂O₃,Nd₂O₃, and Ce₂O₃; Pt; and Ru. Activated carbon and ferric oxide areparticularly preferred. The catalysts may be employed in lowconcentrations on alumina or similar substrates and placed directly intothe hydrogen process stream either in or near the process heatexchangers.

In certain embodiments the catalyst may be incorporated into amicro-channel or tube-in-tube GH₂ process heat exchangers in counterflowwith the cold helium bypass flows from the AMRR stage(s) to maintain‘equilibrium’ hydrogen continuously as the hydrogen is cooled. Thiscontinuously removes the exothermic heat of conversion at the highestpossible temperatures necessary to maintain very high FOM in the overallliquefier. Thus, certain embodiments of the novel processes and systemscan provide a FOM of at least 0.6, more particularly at least 0.7, andmost particularly at least 0.75.

As mentioned above, the bypass HTF is introduced into a bypass HEX. Thebypass HTF cools the process gas that is also introduced into the bypassHEX. In certain embodiments, the bypass HEX includes at least one orthoH₂ to para H₂ catalyst. The sensible heat in the process gas stream isremoved only via the bypass HEX. In other words, no other heatexchangers are required to remove the sensible heat (as mentioned above,the CHEX only removes the reject heat from a colder AMRR stage and verysmall parasitic heat leaks). As can be seen the approach temperaturedifferential in the bypass HEX is 3 K (e.g., 117K vs. 120K).

1. A process for liquefying a process gas comprising:

introducing a heat transfer fluid into an active magnetic regenerativerefrigerator apparatus that comprises (i) a first module comprising 2 to16 successive layers, wherein each layer comprises an independentlycompositionally distinct magnetic refrigerant material having anindependent Curie temperature and wherein the first layer has thehighest Curie temperature and the last layer has the lowest Curietemperature and (ii) a second module comprising 2 to 16 successivelayers, wherein each layer comprises an independently compositionallydistinct magnetic refrigerant material having an independent Curietemperature and wherein the first layer has the lowest Curie temperatureand the last layer has the highest Curie temperature;

flowing the heat transfer fluid through each layer of the first moduleand each layer of the second module;

diverting a portion of the flowing heat transfer fluid from an outlet ofeach layer of the first module to an inlet of the corresponding Curietemperature layer of the second module, except for lowest Curietemperature layer;

diverting a bypass portion of the flowing heat transfer fluid from thelowest Curie temperature layer of the first module into a bypass flowheat exchanger at a first cold inlet temperature;

introducing the process gas into the bypass flow heat exchanger at afirst hot inlet temperature and discharging the process gas or liquidfrom the bypass flow heat exchanger at a first cold exit temperature;and simultaneously subjecting all of the layers of the second module toa higher magnetic field while all of the layers of first module aredemagnetized or subjected to a lower magnetic field.

2. The process of clause 1, wherein the first module consists of eightlayers and the second module consists of eight layers.

3. The process of clause 1 or 2, wherein the process gas compriseshydrogen or methane and the heat transfer fluid comprises helium.

4. The process of any one of clauses 1 to 3, wherein 1.5 to 26 volumepercent of heat transfer fluid is diverted from each layer of the firstmodule to each corresponding Curie temperature layer of the secondmodule.

5. The process of any one of clauses 1 to 4, wherein the bypass portionconstitutes 1 to 15 weight % of the total heat transfer fluid exitingthe lowest Curie temperature layer.

6. The process of any one of clauses 1 to 5, wherein the bypass flowheat exchanger includes at least one ortho H₂ to para H₂ catalyst.

7. A system comprising:

a first active magnetic regenerative module comprising 2 to 16successive layers, wherein each layer comprises an independentlycompositionally distinct magnetic refrigerant material having anindependent Curie temperature and wherein the first layer has thehighest Curie temperature and the last layer has the lowest Curietemperature;

a second active magnetic regenerative module comprising 2 to 16successive layers, wherein each layer comprises an independentlycompositionally distinct magnetic refrigerant material having anindependent Curie temperature and wherein the first layer has the lowestCurie temperature and the last layer has the highest Curie temperature;

at least one conduit fluidly coupled between the lowest Curietemperature layer of the first module and the highest Curie temperaturelayer of the second module;

a single bypass flow heat exchanger (a) fluidly coupled to the lowestCurie temperature layer of the first module and (b) fluidly coupled to aprocess gas source; and

for each layer of the first module and each layer of the second module,an independent fluid conduit between an outlet of each layer of thefirst module to an inlet of the corresponding Curie temperature layer ofthe second module, except for lowest Curie temperature layer of firstmodule.

8. The system of clause 7, wherein each module consists of eight layersof independently compositionally distinct magnetic refrigerant material.

9. The system of clause 7 or 8, wherein the layers of the first moduleand the second module have Curie temperatures 18-22 K apart betweensuccessively adjacent layers.

10. The system of any one of clauses 7 to 9, wherein the successivelayers in the first module are arranged in descending magneticrefrigerant material mass for each layer from the first layer to thelast layer, and the successive layers in the second module are arrangedin ascending magnetic refrigerant material mass for each layer from thefirst layer to the last layer.

11. An apparatus comprising:

an active magnetic regenerative module comprising multiple successivelayers, wherein each layer comprises an independently compositionallydistinct magnetic refrigerant material having Curie temperatures 18-22 Kapart between successively adjacent layers, and the layers are arrangedin successive Curie temperature order and magnetic refrigerant materialmass order with a first layer having the highest Curie temperature layerand highest magnetic refrigerant material mass and the last layer havingthe lowest Curie temperature layer and lowest magnetic refrigerantmaterial mass.

12. The apparatus of clause 11, consisting of eight layers ofindependently compositionally distinct magnetic refrigerant material.

In view of the many possible embodiments to which the principles of thedisclosed invention may be applied, it should be recognized that theillustrated embodiments are only preferred examples of the invention andshould not be taken as limiting the scope of the invention.

What is claimed is:
 1. A process for liquefying a process gascomprising: introducing a heat transfer fluid into an active magneticregenerative refrigerator apparatus that comprises a single stagecomprising dual regenerators located axially opposite to each other,wherein the apparatus comprise (i) a first top regenerator comprising 2to 16 successive layers, wherein each layer comprises an independentlycompositionally distinct magnetic refrigerant material having anindependent Curie temperature and wherein the first layer of the topregenerator has the highest Curie temperature and the last layer of thetop generator has the lowest Curie temperature and (ii) a second bottomregenerator comprising 2 to 16 successive layers, wherein each layercomprises an independently compositionally distinct magnetic refrigerantmaterial having an independent Curie temperature and wherein the firstlayer of the bottom regenerator has the lowest Curie temperature and thelast layer of the bottom regenerator has the highest Curie temperature;flowing a main flow of the heat transfer fluid successively through eachlayer of the first top regenerator and each layer of the second bottomregenerator; diverting a portion of the main flow of the heat transferfluid from an outlet of each layer of the first top regenerator to aninlet of the corresponding Curie temperature layer of the second bottomregenerator, except for the lowest Curie temperature layer of the firsttop regenerator; diverting a bypass portion of the main flow of the heattransfer fluid from the lowest Curie temperature layer of the first topregenerator into a single bypass flow heat exchanger at a first coldinlet temperature; introducing the process gas into the bypass flow heatexchanger at a first hot inlet temperature and at a counterflow with thebypass portion flow, and discharging the process gas or liquid from thebypass flow heat exchanger at a first cold exit temperature; andsimultaneously subjecting all of the layers of the second bottomregenerator to a higher magnetic field while all of the layers of firsttop regenerator are demagnetized or subjected to a lower magnetic field.2. The process of claim 1, wherein the first top regenerator consists ofeight layers and the second bottom regenerator consists of eight layers.3. The process of claim 1, wherein the process gas comprises hydrogen ormethane and the heat transfer fluid comprises helium.
 4. The process ofclaim 1, wherein 1.5 to 26 volume percent of the main flow of the heattransfer fluid is diverted from each layer of the first top regeneratorto each corresponding Curie temperature layer of the second bottomregenerator.
 5. The process of claim 1, wherein the bypass portionconstitutes 1 to 15 weight % of the total heat transfer fluid exitingthe demagnetized lowest Curie temperature layer.
 6. The process of claim1, wherein the bypass flow heat exchanger includes at least one ortho H₂to para H₂ catalyst.
 7. The process of claim 1, wherein the process gasis hydrogen gas and wherein the bypass portion of the flowing heattransfer fluid continuously cools the hydrogen gas from 280 K to 120K inthe bypass flow heat exchanger.
 8. The process of claim 1, wherein theprocess gas comprises at least 99% hydrogen.
 9. The process of claim 1,wherein each layer has a Curie temperature that differs by 18 to 22 Kfrom each adjacent layer.
 10. The process of claim 1, wherein the bypassportion of the flowing heat transfer fluid continuously cools theprocess gas in the bypass flow heat exchanger throughout theliquefication process.
 11. The process of claim 1, further comprisingrepeating the process in at least one additional stage that is fluidlycoupled to the first stage.
 12. The process of claim 11, wherein theprocess gas is hydrogen gas and the first stage cools the hydrogen gasfrom 280 K to 120 K, and a second stage cools the hydrogen gas from 120K to 20 K.
 13. The process of claim 1, wherein volume percent of themain flow of the heat transfer fluid that is diverted from each layer ofthe first top regenerator to each corresponding Curie temperature layerof the second bottom regenerator successively increases per each layer.14. The process of claim 4, wherein the process gas comprises at least99% hydrogen.
 15. The process of claim 14, wherein the bypass portionconstitutes 1 to 15 weight % of the total heat transfer fluid exitingthe demagnetized lowest Curie temperature layer.
 16. The process ofclaim 1, wherein the heat transfer fluid comprise helium.
 17. Theprocess of claim 15, wherein the heat transfer fluid comprise helium.18. The process of claim 15, further comprising repeating the process inat least one additional stage that is fluidly coupled to the firststage.
 19. The process of claim 1, wherein each magnetic refrigerantmaterial operates at or below its respective Curie temperaturethroughout an entire active magnetic regeneration cycle.
 20. The processof claim 19, wherein each magnetic refrigerant material is aferromagnetic material.